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The continuing nitrogen (N) deposition observed worldwide alters ecosystem nutrient cycling and ecosystem functioning. Litter decomposition is a key process contributing to these changes, but the numerous mechanisms for altered decomposition remain poorly identified. We assessed these different mechanisms with a decomposition experiment using litter from four abundant species ( Achnatherum sibiricum , Agropyron cristatum , Leymus chinensis and Stipa grandis ) and litter mixtures representing treatment-specific community composition in a semi-arid grassland under long-term simulation of six different rates of N deposition. Decomposition increased consistently with increasing rates of N addition in all litter types. Higher soil manganese (Mn) availability, which apparently was a consequence of N addition-induced lower soil pH, was the most important factor for faster decomposition. Soil C : N ratios were lower with N addition that subsequently led to markedly higher bacterial to fungal ratios, which also stimulated litter decomposition. Several factors contributed jointly to higher rates of litter decomposition in response to N deposition. Shifts in plant species composition and litter quality played a minor role compared to N-driven reductions in soil pH and C : N, which increased soil Mn availability and altered microbial community structure. The soil-driven effect on decomposition reported here may have long-lasting impacts on nutrient cycling, soil organic matter dynamics and ecosystem functioning.

In nature, grasses simultaneously establish multiple symbiotic associations with endophytic fungi and arbuscular mycorrhizal fungi (AMF). The effect of these multiple interactions on competitive interactions between plants remains poorly understood. In this study, we tested whether endophytes and AMF (Glomus mosseae or Glomus etunicatum) alter plant competition between a subordinate plant species that associates with both symbionts (Achnatherum sibiricum) and a dominant plant species, Stipa grandis, that only associates with one symbiont (AMF). And we hypothesized that endophytes can facilitate the coexistence of the subordinate plant species (A. sibiricum) and the dominant plant species (S. grandis). The results demonstrated that endophyte infection significantly enhanced the competitive ability of the subordinate plant species compared to the dominant plant species. The effects of AMF on plant competition were variable and depended on the identity of the AMF species. Glomus etunicatum gave A. sibiricum plants a higher competitive ability, while G. mosseae gave S. grandis a higher competitive ability. Simultaneous infections of both endophytes and AMF in A. sibiricum also altered the competitive relationships with S. grandis. In conclusion, these results suggest that endophytic fungi can facilitate the coexistence of a subordinate plant species with a dominant plant species. Moreover, endophytes could not only affect the competitive ability of the host plant directly but also indirectly by interacting with different AMF to change the growth of competing plant species. A plain language summary is available for this article. Plain Language Summary

Background and aims Grazing drives carbon (C) and nitrogen (N) dynamics of grasslands through livestock trampling, defoliation, and excretion. Still, the responses of N uptake by plant species and simultaneous C allocation into the soil to grazing intensity remain unclear. Methods In-situ  15 NH 4 + / 15 NO 3 − and 13 C-CO 2 labeling experiment was conducted in Inner Mongolia grasslands under 5 years of grazing with no, light (4 sheep 1.33 ha −1 ) and heavy (12 sheep 1.33 ha −1 ) intensity to reveal the contribution of plant-derived C into the soil and the fate of N on day one and three after 13 C-labeling. Experiment had a completely randomized design ( n  = 3), and every plot included Leymus chinensis , Carex korshinskyi , Cleistogenes squarrosa , and Stipa grandis . Results Grazing increased plants’ total N uptake compared to control (no grazing); higher NO 3 − uptake was found compared to NH 4 + (aboveground: 0.40–20.78 vs. 0.32–6.58 µg N m −2 ; belowground: 0.04–9.92 vs. 0.01–0.49 µg N m −2 ), irrespective of grazing intensity. C. korshinskyi showed the highest N uptake (3–21 µg N m −2 ) under the three grazing intensities. 13 C-CO 2 assimilation was the lowest under heavy grazing (aboveground: 1.06–10.67 mg C m −2 ; belowground: 0.25–1.53 mg C m −2 ) regardless of plant species. 13 C-CO 2 assimilation by L. chinensis and C. squarrosa decreased 3–5 times with grazing intensity. Grazing increased 13 C-SOC irrespective to soil depth compared to no grazing. Conclusions Grazing patterns affected the plants’ total assimilation C capacity and N uptake and the response varies among plant species, as well as the allocation of plant-C transfer into the soil.

Leymus chinensis and Stipa grandis are two important plant species of temperate steppes in Inner Mongolia of North China. They differ in their life forms, e.g., L. chinensis is a type of rhizomatous clonal grass, whereas S. grandis is a type of tussock grass. Here we hypothesize that both plant species possess distinct nitrogen (N) acquisition strategies for their growth and survival. To test this hypothesis, we conducted a four-factor experimental field study using a short-term (three hours) 15 N labeling technique in two plant communities mono-dominated by L. chinensis and S. grandis of the temperate steppes over two months (July and August) and at two soil depths. In both of communities, L. chinensis and S. grandis directly absorbed all three of the common forms of N, including substantial portions of N-derived from glycine (organic and inorganic forms) ranged from 2.7 to 17.8 %, although they absorbed more inorganic N. Nitrogen uptake rates showed significant effects of communities, months, soil depths, and N forms. The uptake rate was higher in August than in July and at 0–5 cm than at 5–15 cm soil depths. L. chinensis and S. grandis showed different preference on N form across months. L. chinensis shifted its uptake pattern from more nitrate (NO 3 − ) in July to more ammonium (NH 4 + ) in August, whereas S. grandis took up comparable NH 4 + and NO 3 − in both months. In general, L. chinensis showed a more flexible N acquisition strategy and S. grandis performed a more concentrated and relatively more stable N acquisition strategy. The distinct N acquisition strategies used by L. chinensis and S. grandis varied greatly across different months and soil depths. These findings are more helpful in further understanding the plasticity of nutrient utilization issues of different plant species in response to N-limited conditions of grassland ecosystems.

The major aims of this study were, firstly, to analyse the grazing-induced steppe degradation process and, secondly, to identify an efficient and sustainable grazing management system for the widely degraded Inner Mongolian typical steppe ecosystem. From 2005-2008 a grazing experiment was conducted to compare two grazing management systems, the Mixed System (MS) and the Traditional System (TS), along a gradient of seven grazing intensities, i.e. ungrazed (GI0), very-light (GI1), light (GI2), light-moderate (GI3), moderate (GI4), heavy (GI5), and very-heavy (GI6). Each grazing intensity treatment was considered a production unit comprising two adjacent plots, one for hay-making (single-cut system) and one for grazing. Hay-making and grazing alternated annually in the MS, while in the TS the same plots were used either for hay-making or for grazing. Effects of management system, grazing intensity, and year on end-of-season standing biomass (ESSB), aboveground net primary production (ANPP), relative difference in ANPP between 2005 and 2008 (ANPPDiff), relative growth rate (RGR), and sward characteristics (litter accumulation, soil coverage) were analysed. Litter accumulation of production units was affected by grazing intensity (P < 0.001) and decreased from GI0 to GI6 by 83%. Correspondingly, soil coverage decreased (P < 0.001) from GI0 to GI6 by 43%, indicating an increased vulnerability to soil erosion. We found varying compensatory growth responses to grazing intensity among years, probably because of temporal variability in precipitation. The ability of plants to partially compensate for grazing damage was enhanced in years of greater seasonal precipitation. The ANPP of production units was negatively affected by grazing intensity and decreased from GI0 to GI6 by 37, 30, and 55% in 2006 (P < 0.01), 2007 (P < 0.05), and 2008 (P < 0.001), respectively. The effect of management system × grazing intensity interaction on ANPP (P < 0.05) and ANPPDiff (P < 0.05) suggested greater grazing resilience of the MS as compared to the TS at GI3 to GI6.

Background and aimsClonal integration between ramets under heterogeneous environment has crucial implications for the clonal plants, is widely distribute in the arid ecosystems because it helps to transfer water and nutrients from the habitats with high moisture or fertility to lower ones. How the clonal integration affects the plant productivity and nutrient uptake under heterogeneous environment still remains unclear.MethodsLeymus chinensis and the neighbouring Stipa grandis grew at two soil moisture environments (homogeneous water content: 8% in both the donor and recipient compartments; or heterogeneous water content: 16% in the donor and 8% in the recipient compartments) and two root connections (connected or severed L. chinensis). After 4 weeks of growth, the plants of donor L. chinensis were labelled with either 15NH4+ or 15NO3−.ResultsThe biomass of recipient L. chinensis and S. grandis and N uptake rate by S. grandis were larger under heterogeneous water water content with connected roots than that with severed roots. The NO3− uptake rate was 40 times faster than that of NH4+ by all the plants irrespective of soil moisture condition and root connection. Consequently, clonal integration increased N translocation from donor to recipient ramets and subsequent N utilization by neighbouring S. grandis. Unexpectedly, NH4+ uptake by recipient L. chinensis with severed roots was 1.5 times faster than that with connected roots under homogeneous environment, this largely ascribed to that the translocation of NH4+ from donor to recipient L. chinensis through common mycorrhizal networks (CMNs).ConclusionsClonal integration increases plant biomass and N uptake in heterogeneous environment and weakens them in homogeneous environment. Plants prefer direct NO3− uptake, whereas arbuscular mycorrhizal fungi preferentially translocate NH4+ to recipient clonal ramets. These findings indicate that clonal plants rely on the clonal integration of clonal roots and CMNs to acquire N from soil.

AimsPlant stoichiometry is known to influence ecological processes and element cycles in ecosystems, which in turn can all be affected by ongoing climate change. While previous studies mainly focused on warming, drought or species invasion, effects of changing water supply on plant stoichiometry have not been well explored.MethodsTo study how water supply affects plant stoichiometry (here C:N, N:P), and whether such effects differ among plant species, a manipulative experiment was conducted in which four grass species (Leymus chinensis, Stipa grandis, Artemisia frigida and Potentilla acaulis) dominant in the Inner Mongolia steppe were subjected to a gradient of water supply via changes in growing-season rainfall.ResultsWater supply significantly impacted C:N and N:P, and these effects differed among grass species. Specifically, while C:N of A. frigida and P. acaulis was unaffected by water supply, C:N of L. chinensis and S. grandis increased with increasing precipitation. Furthermore, N:P of A. frigida showed a unimodal pattern along the imposed precipitation gradient. Whereas aboveground and belowground N:P showed similar trends (but different patterns) with changing water supply, this was not the case for aboveground and belowground C:N. As a result, plant stoichiometry between aboveground and belowground parts followed an allometric pattern.ConclusionsChanges in water supply can significantly modulate plant stoichiometry. These results could improve our understanding of the dynamics of grasslands under climate change.

Climate changes are altering precipitation to more frequent extreme precipitation events that have strong impacts on the structure and functions of grassland ecosystems. We conducted a rain simulation experiment combined with in situ 15 N labeling of three nitrogen (N) forms (NO3−, NH4+, glycine) to investigate how the frequency of extreme precipitation influences plant productivity and N acquisition (N uptake, 15 N recovery, and preference for N form) by the dominant species Stipa grandis and soil microorganisms in the temperate steppe. Extreme precipitation had three frequencies (1, 3, and 6 events for low, medium, and high frequency) with the same total rain amount in 1-month cycle. The low frequency reduced the S. grandis biomass by 39%, whereas the high ones raised the S. grandis biomass by 43% and increased plant and microbial N uptake up to 6.3-fold and 5.1-fold of those under ambient precipitation, respectively. Plants preferred NO3− and microorganisms preferred NH4+ under low frequency, but they showed similar preference for three N forms, leading to chemical niche overlap for NO3−, NH4+, and glycine under high frequency. This indicated that high precipitation frequency effectively reduced the proportion of each N form, which plants and microorganisms competed for as the available N pool increased. Overall, the increase of precipitation frequency (decreasing intensity) shifted the extreme (low frequency but high intensity) to optimal conditions for plant productivity and N acquisition by plants and microorganisms in the temperate steppe. These findings provide new insights for understanding the diverse responses of ecosystem functions to extreme climate events.

Both the dominance and the mass ratio hypotheses predict that plant internal nutrient cycling in ecosystems is determined by the dominant species within plant communities. We tested this hypothesis under conditions of extreme drought by assessing plant nutrient (N, P and K) uptake and resorption in response to experimentally imposed precipitation reductions in two semiarid grasslands of northern China. These two communities shared similar environmental conditions, but had different dominant species—one was dominated by a rhizomatous grass (Leymus chinensis) and the other by a bunchgrass (Stipa grandis). Results showed that responses of N to drought differed between the two communities with drought decreasing green leaf N concentration and resorption in the community dominated by the rhizomatous grass, but not in the bunchgrass-dominated community. In contrast, negative effects of drought on green leaf P and K concentrations and their resorption efficiencies were consistent across the two communities. Additionally, in each community, the effects of extreme drought on soil N, P and K supply did not change synchronously with that on green leaf N, P and K concentrations, and senesced leaf N, P and K concentrations showed no response to extreme drought. Consistent with the dominance/mass ratio hypothesis, our findings suggest that differences in dominant species and their growth form (i.e., rhizomatous vs bunch grass) play an important nutrient-specific role in mediating plant internal nutrient cycling across communities within a single region.

Large herbivores have pronounced effects on nutrient cycling in grasslands. These organisms are known to alter the quality and quantity of plant production as well as the amounts and quality of plant litter and animal wastes. The generalization that the relative quality of detritus inputs is enhanced by herbivores is well known, but how this process is affected by diet selection processing and feces production of different large herbivores remains largely unstudied. Here, we measured how these differences for cattle and sheep on a dry grassland might influence nitrogen (N) mineralization from feces. We found that cattle of larger body size tended to select the low quality grass Stipa grandis as their major food source. In contrast, the subdominant grass Leymus chinensis, with relatively high N content, was a majority in the diet of smaller sheep, when palatable forbs were insufficient in the field. This diverse diet quality resulted in a C:N ratio of cattle feces that was higher than that of sheep feces. Relatively higher labile C availability in the cattle feces, namely relatively higher cellulose/hemicellulose contents, promoted microbial growth and in turn accelerated cattle feces decomposition. A surprise finding was that the feces from cattle mineralized about twice as much N as feces from sheep, despite the latter having slightly higher N content. From a grassland productivity perspective, increasing the proportion of large body-sized species in grazing herbivore assemblages perhaps is beneficial to forage productivity and nutrient recycling by the rapid degradation of feces.